1. Field of the Invention
The present invention relates to a shift control system and method for an automatic transmission in a vehicle; and more particularly, to a shift control system and method for reducing shift shock in an automatic transmission when shifting from reverse to drive.
2. Discussion of Related Art
Automatic transmissions make it possible for unskilled drivers to quickly and easily learn to operate a vehicle. The provision of a torque converter between the engine and the automatic transmission eliminates the need for a driver to learn how to use a clutch pedal for engaging and disengaging the transmission. Furthermore, the control system for the automatic transmission automatically decides when to shift up and down, thus eliminating the need for the driver to learn these skills.
FIG. 1 illustrates a conventional hydraulic control system for an automatic transmission which includes a front clutch 100, rear clutch 110, low reverse brake 120, end clutch 130 and kickdown band brake 140 (i.e., the friction elements). FIG. 1 also illustrates the torque converter TC that is coupled between the engine and the automatic transmission. The conventional hydraulic control system as shown in FIG. 1 includes shift solenoids S1 and S2 which control the flow of hydraulic fluid to the front clutch 100, rear clutch 110, low reverse brake 120, end clutch 130 and kickdown band brake 140 to effect gear change operations. A pressure control solenoid S3 controls the pressure in the hydraulic system, and thus how quickly the various friction elements are engaged and disengaged. A transmission or shift control unit (TCU) controls the operation of the shift solenoids S1 and S2 and the pressure control solenoid S3.
As further shown in FIG. 1, in conjunction with the shift solenoids S1 and S2, the flow of hydraulic fluid to the friction elements is also controlled in accordance with the position of a driving mode select lever 150. An operator uses the driving mode select lever 150 to place the vehicle in park P, reverse R, neutral N and drive D. The driving mode select lever 150 can also be used to select the second speed stage of drive (as the maximum forward speed stage) D2 or the low speed stage of drive.
When driving the vehicle in reverse and at low speeds, shifting the driving mode select lever 150 from reverse to drive does not cause great shift shock. The reason for this is that the rear clutch 110 is applied when moving from reverse to the first speed stage of drive, and that the application of the rear clutch 110 does not cause a significant instantaneous change in vehicle speed.
However, when driving in reverse at high speeds, and a shift is made into drive, instead of shifting into the first speed stage of drive, the control system attempts to shift into the second speed stage of drive due to the high speed at which the vehicle is travelling. Shifting into the second speed stage of drive requires application of the kickdown band brake 140. Application of the kickdown band brake 140 causes a significant instantaneous change in vehicle speed, such that great shift shock occurs.